1. Field of the Invention
The present invention relates to a battery protection circuit, more particularly to an accurate scan-mode voltage detection circuit.
2. Description of Related Art
It is well known that a Lithium ion battery has been used as a battery cell in a battery pack. The Lithium ion battery generates a high energy density per weight and volume, and generally provides a reduction in size and weight of a portable type apparatus. However, the Lithium ion battery has a safety problem that its performance characteristic is degraded when it is charged by an overvoltage, and what is more, it even tends to become explosive if it is operated beyond its capacity. Hence, every battery cell needs a protection circuit and a design of a high accuracy overvoltage protection circuit becomes important.
The power consumption of a protection circuit is only several microamperes. Each of these ICs usually comprises three voltage detection circuits, a short circuit detection circuit, reference voltage sources, an oscillator, a counter circuit and a control circuit. Normally, there are two implementations.
One is a continuous mode, a working principle of which can be explained by referring to FIG. 1. As shown in FIG. 1, two ladder resistors R3 and R4 are provided for dividing a battery voltage, and a voltage comparator is provided for comparing a divided battery voltage of the two ladder resistors and a reference voltage of the reference voltage source. When the divided battery voltage is higher than the reference voltage, the voltage comparator outputs a low level to enable the oscillator and the internal control circuit. After counting a predetermined number of clocks, the control circuit generates a control signal COUT to prohibit the charge of the battery.
The continuous mode means that the voltage comparator, the voltage divider and the reference voltage are always on. To reduce the power consumption, it demands that every current path in the protection circuit is very small (e.g. many current paths flow in dozens of nano-amperes) and all transistors stay in a sub-threshold region. Taking a current mirror as an example, the output current is exponentially depending on a gate-source voltage. The demand for the process model is very critical, and the circuit operating under the sub-threshold region is weak and not reliable. Generally, they are susceptible to process variation and noise interference.
The other method is a scan mode. The oscillator is always on and the system detects various abnormal conditions, such as overcharging, overdischarging and overcurrent one by one for some clocks. The voltage detectors are turned on only once for several clocks. At other times, the voltage detector is turned off to save power consumption.
However, the voltage of node's operation point in the voltage detector is noncontinuous in the scan mode. In off-time of one period, gates of p-type MOS transistors in the voltage detection circuit or commonly are pulled high to the power supply, and gates of n-type MOS transistor in the voltage detector commonly are pulled low to the ground so that the voltage detector is disabled for saving power consumption. In on-time of one period, the gates of p-type MOS transistors in the voltage detector are pulled low to the working voltage from the power supply, and the gates of n-type MOS transistor in the voltage detector are pulled high to the working voltage from the ground so that the voltage detector is enabled for properly operation. It takes too much time to recover the proper operation of the voltage detector from the off-time state and reduces the normal operating time of the voltage detector. For the reason above, there is an offset between a transient overvoltage threshold and a DC threshold. Moreover, because the delay time is affected by process and temperature etc, this would impact the accuracy of the overvoltage protection threshold.
Thus, improved techniques for an accurate voltage detection circuit are needed.